Receptor tyrosine kinases (RTKs) are the targets of numerous therapeutic agents used in the treatment of cancer and other diseases. These agents include small molecule tyrosine kinase inhibitors (TKIs) that inhibit the intracellular catalytic activity of RTKs and antibody therapeutics that target the extracellular ligand-binding domain. Many of the 58 RTKs in the human proteome have now been implicated in cancer and other diseases - either as oncogenic drivers or in mechanisms of resistance to existing targeted therapies. Accordingly, efforts are underway to extend approaches that have been successful for EGFR, for example, to other RTKs. Here we focus on the Tie1/2 and the Tyro3/Axl/Mer (TAM) families of RTKs as examples of the 40% of human RTKs that contain membrane proximal fibronectin type III (FNIII) domains in their extracellular region. The mechanisms of ligand-induced activation for these two families is poorly characterized. Existing structural studies have failed to reveal the mechanism of ligand- induced receptor activation. For Tie2 we find that the membrane-proximal FNIII domains play a key role in receptor dimerization and activation. In this proposal, we will investigate whether the membrane proximal FNIII domains in the Tie1/2 and TAM families of RTKs share a common mechanistic role. In our specific aims we will address these questions: 1. How do the membrane-proximal fibronectin type III (FNIII) domains of Tie2 regulate receptor activation states? 2. Is there a common function for membrane proximal FNIII domains in RTKs? These studies will provide important insight into the regulation of these two families of RTKs, enriching our appreciation of extracellular regulation of RTKs in general, and opening potential new avenues for intervention where these receptors are aberrantly activated in cancer and other disease states.